Electrical apparatus employing piezo-electric devices



Feb. 21, 1933. v J. ROBINSON. 1,898,895

ELECTRICAL APPARATUS EMPLOYING PIEZO ELECTRIC DEVICES Filed June 9, 1930 v Patented Feb. 21, 1933 ilNl ED STATES JAMES nonmson, or mn'non, ENGLAND BRITISH BADIOSTAT CORPORATION,

ASSIGNOR, BY MESNE ASSIGNMENTS, 'I'O LmITED,,A. CORPORATION OF CANADA ELEGLRIGAI: APPARATUS EMPLOYING PIEZO-ELECTBIC DEVICES Application filed June 9, law, Serial No. 460,054, and in Great Britain July 26, 1928.

This invention is for improvements in'or relatin to electrical apparatus employing piezo-e ectric devices.

The invention is concerned with receiving apparatus for electro-magnetic waves in which a piezoelectric device is employed to provide a high degree of select vity. The piezo-electric device being susc'eptlble to very small changes of frequency, 1t will be apparent that even small values of nherent capacity of the crystal or 1ts mounting may impair the efiiciency of the apparatus. ThlS also applies to the inherent nductance and resistance. An object of thls invention is to provide means to obviate this disadvanta e,

according to the invention a receiver for electro-magnetic waves comprises a plezoelectric device, a thermionic valve, one electrode of which is directly connected to one electrode of the iezo-electric device, and means for controlling the amount of energy that is passed otherwise than through the crystal piezo-electric device, so as to get least interference.

In one form of the invention a receiver for electro-magnetic waves comprises a piezo-electric device, a thermlomc valve, one electrode of which is directly connected to one electrode of the piezo-electric device, and a capacity for controlling and/or balancing the energy that is passed otherwise than through the piezo-electric crystal, so as to get least interference.

Further features of the invention will be apparent from the following description taken in connection with the accompanying drawing, in which:

Figure 1 is a diagrammatic illustration of one form of receiver embodying the princi les of the invention; and

igure 2 is a chart showing a recelver resonance curve and illustrating the manne in which interference is avoided.

Referring s ecifically to Fi 1 of the drawing, it be noted that t e incoming energy is applied to the terminals 10 to feed a coil 11 which is electromagneticall coupled to another coil 12; alternative y,. an electro-static coupling may be employed.

This coil 12 is tuned by two condensers 13 connected in series across the coil and the connection between the condensers is joined to the cathode of a thermionic valve 14.

One end of the coil 12 isalso connected to one electrode of a piezo-elec'tric device 15 (such as a quartz crystal mounted between electrodes as shown) the other electrode being connected directly to a control electrode of the valve 14. .The other end of the coil 12 is connected through a controlling and/or balancing condenser 16, which is adjustable, to the same control electrode as'shown or to another such electrode of the valve 14. The arrangement of the condensers 13 and 16 and the piezo-electric device 15 thus constitutes an electrical bridge which may be balanced for capacity by adjustment of the condenser 16. The anode circuit of the valve includes output terminals 17 and a battery 18, and the valve is arranged to operate as a detector by the provision of a grid resistance 19 and bias battery 20.

The piezo-electric device 15 and the tuned circuit 12 and 13, 13, are both tuned to the 75 frequency of the incoming energy, and one mode of operation is first to adjust the con denser 16 to provide a balance for the static capacit of the piezo-electric device and then to re-a just the condenser 16 to reduce any 80 other interference such as that causing an undesirable heterodyne effect.

The theoretical considerations underlying this method of receiving signals may be more clearly understood by reference to Figure 2 85 of the drawing in which a resonance curve 25 of a selective receiver is shown, the ordinates representing the response of the receiver and the abscissae the signal frequency. The dash and dot line 26 represents the energy by-passed through the condenser 16, it being observed that the condenser is nonresonant and is therefore sensibly independent of the frequency within the relatively small range of the resonance curve of the receiver. It will be noted that the phase of this energy is shown as reversed on passing through the resonant frequency of the receiver indicated by the line AA. This has been done merely for convenience in illus- I bined with the ener tra commg' si al which changes phase in assing through the resonant point. It ca ii be shown, for instance, that in a mechanical resonator such as a iezo-electric crystal, the

base of the force vibrations therein roueed b an exciting electromotive orce changes lzy 180 at the resonant point. When the exciting frequency is lower than the frequency of the crystal, the forced vibrations lead the exciting frequency, whereas the forced vibrations will lag when the exciting frequenc is higher than the natural frequency of t e crystal. This is equally true of any tuned circuit.

Thus, by indicating a phase reversal of the energy by-passed through the'condenser 16, the actual efiect can 0 more clearly shown and the resonance curve of the receiver may assume the conventional form.

It will be noted that when the energy, which is represented'by the line 26, is compassing through the crystal in opposed p ase, the resultant energy response of the receiver will follow ap proximately the dash line 27 and the resonance curve of the receiver will be distorted "or shifted slightly so as to afford greater selectivit as regards signals of a lower frequency t an the resonant frequency of the receiver at a slight sacrifice of selectivity as regards si als of higher frequency than the resonant uency. Thus, if a heterodyne note results from interference between a wanted signal and a signal of lower frequency, the ca acit 16 may be increased to an amount s ight y ater than that reuired to balance out t e capacity effect of e crystal and holder and the distortion of the resonance curve will be sufiicient to elim inate or substantially eliminate the resulting beat note.

On the other hand, if the capacity of the crystal is decreased to an amount less than that required to balance the crystal capacity, the resonance curve of the receiver will be shifted to the other side so that the interference between the wanted signal and a signal of slightly higher frequency may be eliminated. Thls will be evident from the fact that the energy passing through the crystal by reason of the capacit of the holder will be of reversed phase rom that which passes through the condenser 16 and thus the line 26 representing the additional energy will be below the datum line for the lower frequencies and above this line for the higher fre uencies.

The device s own may be combined with amplifiers preceding and following it and by way of example the device may be emp oye in the intermediate frequenc stage of a super-heterodyne-receiver. so the valve 14 may be operated as an amplifier,

the effect since, as a matter of fact,- it is t e response of the receiver to the inq with a separate detector following it if reuired.

It will be understood that the invention is not restricted to the precise details of the specific example shown in the drawin for instance, two valves ma be employe with their cathodes connecte together, a control electrode of one valve connected to the piezoelectric device, a control electrode of the other valve connected to the condenser 16 and an output circuit common to both valves. Furt ermore, a quartz crystal without piezo-electric effect may be employed as a dielectric for the condenser 16.

I claim 1. In the art of receiving wave form signals the method of eliminating a heterodyne note resulting from interference between two signals of closely adjacent frequencies which comprises selecting, from the received s' al energy, the energy which is mainly o the frequency of one of said signals, separating a relatively small amount of the total received signal energy and combining said small amount of energy in base opposition with the said selected signal energy.

2. In the art of receiving wave form signals the method of eliminating a heterodyne note resulting from interference between two signals of closely adjacent frequencies which comprises selecting, from the total received signal energy, the energy which is mainly of the frequency of one of said signals but also an unwanted portion of ener of the frequency of the other of said sign s, separating a re atively small amount of the total received signal energy and combining this small amount of ener in phase opposition with the said selecte signal energy to balance out the said unwanted portion of this selected energy.

3. In the art 0 receiving wave form signals with a piezo-electric resonant device having shunt ca acity, the method of eliminating a hetero yne note resulting from interference between two si als of closely adjacent frequencies whic comprises selectin from the total received signal ener by sai iezo-ele'ctric device the energy whic is main y of the frequency of one of said signals and an unwanted portion of energy of the frequency of the other of said signals, bypassin a portion of the total received ener throug a capacitative path of lesser impe ance than the capacitative impedance of the iezo-electric device, and combining the resu tant energy in op osite phase to balance out the unwante lected energy.

4. In the art of receiving wave form signals with a piezo-electric resonant device having shunt capacity, the method of eliminating a heterodyne note resulting from interference between two signals of closely adjacent frequencies which comprises selecting portion of the se- 7 from the total received signal energy by said piezo-electric device the energy which is mainly of the frequency of one of said signals and an unwanted portion of energy of the frequency of the other of said signals, by-passmg a portion of the total received energy throug a capacitative path of greater impedance than the capacitative impedance of the piezo-electric device, and combining the resulant energy in opposite phase to balance out the unwanted portion of the selected energy.

In testimony whereof I have signed my name to this specification.

JAMES ROBINSON. 

